Looking for Member Chevytech

[72CSTC5]

My dad bought an 83 gmc that had a blown motor. Someone had put in an 95 suburban motor so we pulled it and went and bought a crate general motors 350 engine and installed that. Put new plugs and wires and rotor. The problem is that the truck has a miss at idle. He has all of the vacuum ports that he is aware of blocked off. Any suggestions on where to look next. Could the distributor be in one tooth off causing this miss? Thanks in advance for your help.

[ChevyTech]

Could the distributor be in one tooth off causing this miss?

If you got it running and were able to set the timing the distributor should be good enough. Did you set the timing?
I am assuming it is carbureted like a 1983 and not fuel injected like a 1995.
You should go back a double check the firing order and the plug wires making sure all are correct.

The problem is that the truck has a miss at idle.

Does it only miss at idle and runs ok otherwise?
If it only misses at idle you should look for a vacuum leak. Did you have any trouble with the intake gaskets or putting the intake manifold on?
If it misses all the time the engine could have a valve adjusted wrong (to tight). You could check the compression on the cylinder that misses. Did you figure out which cylinder it is yet? It would be a good idea to make sure you did not crack a spark plug.

If you know which cylinder is giving the problem, you could swap spark plug and wire with another cylinder and see if the misfire moves.

[72CSTC5]

They are all new spark plugs. I was thinking the same thing too about a valve being adjusted too tight from the factory or possibly having a vacuum leak. I tried to run all of the vacuum hoses according to the diagram on the core support but it looks like some parts are missing so my dad just plugged them all or so he thinks. It only misses at idle. The other times it is running fine. I do not know how to check to see what cylinder is missing. It starts fine and the timing is set at 4 degrees BTDC at idle with the vacuum advance disconnected. When you reconnect the advance the timing goes to only 11-14 degrees. Is that enough? The intake question is interesting because I recall my dad telling me that when he bought the intake gaskets there were none for the front and rear so he filled with RTV and put the intake on. He also said he has a small oil leak dropping down on to the starter that he can not see where it is coming from. The motor has less than 5 miles on it probably.

[ChevyTech]

Being there is only a miss at idle, it sure sounds like a vacuum leak.

I like to use a needle nose pliers to pinch off vacuum hoses, one and at a time, and listen for a difference.
I also use a 3 foot long piece of heater hose to listen for a hiss as I move it around the engine.

Vacuum can leak through components such as the brake booster or charcoal canister purge valve.
Don’t over look the possibility of the EGR valve being stuck open a little.

Look at the edge of the intake manifold and see if the gasket looks uniform and in place.

The intake question is interesting because I recall my dad telling me that when he bought the intake gaskets there were none for the front and rear so he filled with RTV and put the intake on.

That’s the way they are done on 1980s V8s. (No front and rear)